1. Field of the Invention
The present invention relates in general to a cylindrical high voltage supercapacitor and a method of manufacturing the same. More particularly, the invention relates to a spirally wound cylindrical high voltage supercapacitor with at least one bipolar electrode interposed between an anode and a cathode, and a method of manufacturing the same.
2. Description of the Related Art
Batteries are always demanded for portable electronic devices, cordless power tools, uninterruptible power systems, and electric vehicles. There is a persistent trend that the batteries in the preceding applications should be compact, light, and high in energy-content. In order to meet these requirements, new electrode materials are explored and new cell designs are developed for batteries. Energy stored in batteries can be released in time periods from seconds to hours. When a long discharge time is needed, batteries require large energy densities that are generally achieved by storing a large amount of reactants in a given volume of electrodes of novel materials. In applications in need of a large current in a short discharge time, the loads will consume a peak power that can only be provided by batteries with high power densities. Power is the product of current density and cell operating voltage whereby the latter parameter is easier to maximize for attaining high power densities. The most efficient way for maximum operating voltage is a bipolar design that can produce a high voltage in a small cell volume. A large number of publications on enhancing the power density of lead-acid by using bipolar design have appeared in the literature, for example, U.S. Pat. Nos. 4,188,464; 4,539,268; 4,254,415; 5,729,891; 5,882,817; and 5,993,494, as well as the report by LaFollette and Bennion; xe2x80x9cDesign Fundamentals of High Power Density, Pulse Discharge, Lead Acid Batteriesxe2x80x9d; J. Electrochemist. Soc., Vol.137, No.12, December 1990; PP.3693-3707 are all on the subject, just to cite a few. Regardless of the endeavors by the battery industry, provision of peak power is preferably furnished by capacitors, particularly by supercapacitors, since the latter intrinsically have higher power densities than all batteries.
Supercapacitors are energy-storage devices that depend on surface adsorption, that is, electrostatic attraction, to accumulate charges up to thousands of farads (F) from surface oxidation-reduction. Because of the rapid physical process of adsorption and shallow accumulation of charges such as superficial reactions, supercapacitors intrinsically have much greater power densities than all batteries can accommodate. Batteries utilize slow chemical reactions that occur in the bulk of electrodes for energy storage, and the release of energy in batteries is equally slow. It is this charge-discharge mechanism that grants batteries with high energy densities, and supercapacitors with high power densities. In the development of the battery or supercapacitor, the goal is to improve the property where the device is deficient. As the energy stored in a supercapacitor is proportional to the square of its working voltage as described in equation (1),
E=1/2CV2xe2x80x83xe2x80x83(1)
where E is stored energy, C is capacitance, and V is working voltage, it is plausible to increase V to enhance the energy density of supercapacitors. In addition to choosing an appropriate electrolyte for augmenting V, for example, an aqueous electrolyte provides 1V while an organic system can provide as high as 3V of working voltage, the bipolar design as used for batteries may be applied to supercapacitors. As a matter of fact, many works have employed such design for attaining high V for supercapacitors as disclosed in U.S. Pat. Nos. 5,450,279; 5,646,815; 5,907,472; 5,930,108; 5,959,830; 5,993,494; 6,187,061; 6,304,426; 6,306,270; and 6,320,740, as well as in Japanese Pat. No. JP-A-6-5467. Just like their bipolar battery analogy, the supercapacitors in the previously cited references are all composed of a stack of bipolar electrodes separated by ionically conductive separators in a hermetically sealed package.
However, it is labor-intensive and costly to pile up layers of electrodes, separators, and sealing gaskets in sequence and in multiple repetitions to form the stacked bipolar supercapacitors. Furthermore, the resulted devices will be bulky and can only be in a rectangular or a square shape, which will deny the use of the devices in applications that are limited in space and configuration.
In the prior art, the working voltage for cylindrical supercapacitors ranged between 2.3V to 2.5V, while most integrated circuits (IC""s) require an operation voltage between 3V to 5.5V. To support operation of an integrated circuit, at least two low voltage supercapacitors connected in series are required. For a printed circuit board (PCB) that requires high capacitance, the capacitor that provides large capacitance and typically occupies a large area such as an aluminum electrolyte capacitor is used. It is thus difficult to miniaturize the printed circuit board.
Accordingly, the present invention provides a cylindrical high voltage supercapacitor and a method of manufacturing the same, which employs a spirally winding method. Therefore, the cylindrical high voltage supercapacitor can be fabricated in a simple process.
The present invention further provides a cylindrical high voltage supercapacitor and a method of manufacturing the same, where the cylindrical high voltage supercapacitor can be formed in a compact volume with a high volume efficiency.
The cylindrical high voltage supercapacitor and the method for manufacturing the same provided by the present invention have reduced number and volume of components, as required by a printed circuit board.
In addition, the cylindrical high voltage supercapacitor provided by the present invention can be connected in parallel for power applications.
The cylindrical high voltage supercapacitor provided by the present invention comprises an anode, a cathode, at least one bipolar electrode and at least one separator. The bipolar electrode is interposed between the anode and the cathode. The separator is intervened between two neighboring electrodes. The above anode, cathode, bipolar electrode and separator are wound together to form a spirally wound concentric roll.
The method of manufacturing a cylindrical high voltage supercapacitor provided by the present invention comprises the following steps. An anode and a cathode are provided. At least a bipolar electrode is interposed between the anode and the cathode. Between each pair of electrodes, a separator is intervened. According to the above-mentioned order, the anode, the cathode, the bipolar and the separator are spirally wound as a roll to form the cylindrical high voltage supercapacitor.
The present invention interposes at least one bipolar electrode between the anode and the cathode and applies a spirally winding process to replace the stacking process. Therefore, the cylindrical high voltage supercapacitor is made in a less laborious process, and is thus more advantageous for automation.
The spiral winding process provides more electrode area with less material in each cell. From the aspect of custom-built shape, the cylindrical shape is more easily modified compared to a rectangle or square. The cylindrical high voltage supercapacitor has a smaller volume and a better volume efficiency.
The voltage of the cylindrical high voltage supercapacitor increases with the number of the bipolar electrodes. Therefore, by increasing the number of the bipolar electrodes, the working voltage thereof can be increased as required.
Further, by increasing the working voltage, a cylindrical high voltage supercapacitor is used to replace two serially connected low voltage supercapacitors. A higher energy density is thus obtained. The traditional large volume component such as aluminum electrolyte capacitor can be replaced to reduce the chip counts of a printed circuit board, which can thus be miniaturized.
In addition, having sufficient working voltage allows the capacitors to be connected in parallel. While being connected in parallel, the total resistance of a module is equal to the resistance of a single component divided by the number of the supercapacitors in the module (assuming that the resistance for each component is the same). The module thus has more power than each component. Further, when one component of the module fails, the module can still operate properly. On the contrary, the serial module stops working when any of the components fails. Therefore, the parallel module is more reliable than the serial module.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.